Building a third runway at Heathrow could lead to unprecedented congestion on London's road, rail and tube networks, transport bosses have warned.

Transport for London (TfL) this week estimated it would cost between £15bn and £20bn to improve transport connections sufficiently to cope with the surge in passengers expansion would bring.

It claimed the Airports Commission, which has recommended expansion of the airport, had only assessed the impact on surrounding roads and public transport up to 2030 - by which time the extra runway is not expected to be operating at full capacity.

"The simple word would be congestion"

Richard De Cani, TfL's director of strategy and policy, told a meeting of the London Assembly's transport committee on Tuesday (November 10) that a third runway could lead to unprecedented congestion across west and south-west London.

"The simple word would be congestion - congestion on the road network, congestion on the rail network of a scale that we haven't seen," he said.

"The level of crowding you would have on those rail corridors into central London would be some of the worst that we currently see in London and that’s based on 2030 demand, the year of opening.

"So it's a level of crowding and congestion that we believe would start to impact quite significantly on the whole performance of the transport network across west and south west London."

Heathrow says a third runway would enable it to increase annual flights from the current cap of 480,000 to 740,000.

The Airports Commission forecast that with a third runway the number of passengers using Heathrow each year would rise from 73 million at present to 125 million by 2030.

It estimated this would lead to around 110,000 extra trips on public transport per day and 41,000 more by car.

However, when the third runway is fully utilised, TfL predicts those figures will increase by a further 37,000 and 32,000 respectively.

"We are confident in our plans..."

A Heathrow spokeswoman said expansion would come with a "step-change" in rail access to the airport, and by increasing the number of passengers taking public transport it would ensure there was no more airport-related traffic than today.

She said all but one of the major improvements to rail connections already had public funding committed, with only the proposed Southern Rail connection requiring additional finance.

"We are confident in our plans because we already have an outstanding record in increasing the use of public transport to get to the airport and encouraging people to shift out of their cars," she added.

"Over the last 20 years, our passenger numbers have risen by almost 80% but airport related traffic has remained static."

He pointed out that Mr De Cani had told the House of Commons' environmental audit committee only last month that was the funding gap for all transport improvements needed across London and not just those which are essential to cope with the demands of a busier Heathrow.

London Assembly member Valerie Shawcross, who chairs the Assembly's transport committee, said: "Before the Government makes its decision on airport expansion, there are big questions to be answered around what transport infrastructure is needed and who will pay for it. We can't allow a bigger Heathrow to clog up London's roads and public transport network."

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